Most mammals, including humans, give birth to live young. In other words, they don’t lay eggs. Many other species such as reptiles and some fish also give live birth. Birds, however, always give birth through the process of laying eggs.
There is a name for creatures that give birth to live young. They are called viviparous animals. Viviparous translates to reproduction inside the body of the mother.
Other animals give birth via the laying of eggs. They are called oviparous animals. Broadly speaking this says that they are animals whose embryos develop outside of the body and within eggs. Animals that fall into this group are most fish, amphibians, reptiles, insects, and of course, birds.
The interesting story is of how birds came to give birth via the laying of eggs. And the answer is rooted in how they developed as a species. Birds are one of the few non-insect animals that have the ability to fly. But they didn’t start out like that.
It is still not well known that many scientists believe that the bird species developed from the dinosaurs. And, as far as we can tell, dinosaurs were earth bound creatures. In a like manner, speculation is that the early birds that developed from dinosaurs did not have the ability of flight either. As the species of birds evolved, however, they gained certain abilities and lost others. In order for birds to fly they had to lose density and weight. Otherwise, there is no way that they could be light enough to be supported by the winds. One of the prime reasons that birds can fly is that they are so lightweight, much lighter than a mammal of the same size.
Now as everyone who has monitored any type of viviparous mammal, as the fetus starts to grow within the mom, it gains weight. This means that the mother gets heavier and heavier by the day. If birds were viviparous, it would not be too long before the combined weight of the mother and fetus resulted in the mother being too heavy to fly. This would prevent her from being able to escape predators, to hunt for many foods, and so on. If, however, the fetus can be allowed to grow outside the body, in an egg, the problem is resolved.
And this is the reason that as soon as she is able to, the female bird will get rid of the egg within her by laying it in a nest where it will be allowed to incubate and hatch.
Birthing young through having them hatch from an egg, however, arrives with its own set of problems. The main problem is that it is harder to protect the unborn. Since the fetus is outside of the body, the female and male pair must work together to protect the egg. This means that someone must always stay close by to prevent predators from taking advantage of a missing parent by raiding the nest and stealing eggs.